How to reduce noise in really dark scene
Unfortunately, there's not a way to get the render less noisy other than increasing the number of samples.  One of the reasons for this is that most of the light in the dark areas is coming from indirect bounces.  There are a few ways to clean it up though.

First, you can use Render Passes:



This allows you to edit individual passes such as Diffuse Indirect to try to blur the noise.  This is pretty effective but it can also smear some detail if you're not careful.  The main problem with this approach is the amount of "noding" you must do to recombine the passes.  The good thing is that you can use Node Groups to make it look nicer [ Can I make a custom node from a group and have that available in other projects? ]

Here's the node tree for this.  It's a lot simpler than it looks.  Basically you just add the direct and indirect pass for each shader and then multiply it by the color pass for that shader.  I made this node tree into a group:



The only purpose of this node group is to combine all the passes into the final image.





Use the Depth and Normal passes to create your Determinator for the Bilateral Blur node like so and feed them into the Node Group above:



At a minimum I usually use one Bilateral Blur node for the Diffuse and Glossy Indirect passes.  This method gives that most control over denoising.





However, in the 2.79 Beta version, there's a new Denoising filter included with Blender that is quite effective: [ https://wiki.blender.org/index.php/Dev:Ref/Release_Notes/2.79/Cycles ]



It can use your GPU to calculate the denoising of tiles, too.  The problem with this method is that it's processed as it renders.  So if you don't like the options you've chosen, you have to render the entire scene again.  That can get old really fast, especially if you have to wait ten minutes per render.  It also adds significant time to your render but not as much as a comparable amount of samples.  There can also be some artifacting in glossy shaders.  I've found that in really noisy scenes (like the one below) that it can look splotchy.  This can probably be improved with fiddling with the settings.

Here's a quick comparison of these two methods with a piece of a scene I've been working on.  It has bump mapped walls and it's very dark, so it can be really noisy.  In my opinion the Render Pass version looks the best and is much more versatile.  But the Denoiser is easier to use.  This scene was rendered at 800 samples.  I've increased the brightness manually so you can see the noise better: